1. Technical Field
The disclosure relates to hetero-junction bipolar transistor (HBT) fabrication and the structure thereof. More particularly, the disclosure relates to fabrication of a HBT using <110> crystalline orientation.
2. Related Art
In the current state of the art, hetero-junction bipolar transistors (HBTs) are fabricated by epitaxially growing desired semiconductor layers to form an emitter layer, a base layer, and a collector layer on a semiconductor substrate. The semiconductor substrate usually has a plane of <100> crystalline orientation as a main surface. HBTs present numerous advantages as high performance transistors because of the different semiconductor materials used for forming the emitter, base and collector therein. The difference in semiconductor materials creates different energy band-gaps that form hetero-junctions therebetween. Graded base junctions, formed with silicon germanium (SiGe) where germanium is graded therein, are of particular interest as resulting drift fields accelerate minority carriers injected from the emitter through the base toward the collector. As a result, carrier transit time is reduced, improving high-frequency performance through increased emitter injection efficiencies, reduced base resistance and base-emitter capacitance. Improved high-frequency performance in HBTs generally permits higher cut-off frequencies over silicon bipolar junction transistors (BJTs).
With increasing demand for high performance transistors, there is a need to further enhance carrier mobility. Improvements on currently known technologies in the fabrication of metal oxide semiconductors (MOS) include fabricating HBTs on silicon-on-insulators (SOIs), which typically has a surface crystalline orientation <100>. However, demands on efficiency of devices incorporating transistors necessitate faster/higher performance and hence greater carrier mobility/speed in the transistors. Currently available HBTs, which are fabricated on silicon (Si) substrates of <100> crystalline orientation have limited carrier mobility, particularly in pnp-type HBTs, which ultimately presents difficulty in the fabrication of complementary bipolar CMOS (CBiCMOS) circuits.